Effect of temperature on electrical resistivity of EVA-LDPE nanocomposites for photovoltaic encapsulating application: An activation energy method

This article evaluates the effect of mixing strategy on thermo-electrical resistance of ethylene vinyl acetate (EVA), low-density polyethylene (LDPE) and fumed silica nanoparticles (FSN) based composite encapsulating materials. The nanocomposites were synthesized by solution compounding method with various sequence of ingredients addition. The quantity of ingredients is same for all nanocomposites where the ratio of EVA, LDPE and FSN addition is 10:1:0.11. The structural morphology of the samples prepared by different mixing sequence was studied under Transmission Electron Microscopy (TEM). DC volume resistivity (VR) were evaluated at 25 °C, 50 °C and 75 °C for all the compositions to check the insulation behaviour at extreme service temperature as VR is an assessing parameter for photovoltaic (PV) encapsulating materials. For obvious reason, the magnitude of VR decreases with increasing temperature. Activation energy (Ea ) was estimated from Arrhenius equation and the value evaluated was very much dependent on the ingredients mixing strategy. Good electrical resistance with desired activation energy were obtained for the nanocomposites; prepared with a particular mixing strategy where percolation threshold achieved. Finally, the TEM structure and insulation property relationship has been addressed against the experimental findings.


Introduction
The market for renewable energy has been increasing remarkably in the last decade due to limitless consumption of fossil fuels.Solar energy is a rich terrestrial renewable source of energy for the sunniest countries.PV module has prominent advancements and highest economical potential among all sustainable energy sources.Encapsulation of PV cell is a compulsory method for safeguarding the crystalline silicon wafer from unwanted environmental and mechanical stresses [1].Encapsulants maintain the electrical insulation between photovoltaic cells and circuit elements and produces current & voltage during service time.It also combines the top glass and rear back sheet with the module and plays an important role for long-term stability.EVA is one of the most common encapsulant material.The encapsulating layer should eliminate the excessive heat of the silicon wafer [2].But the uncontrolled service temperature has significant effect on the EVA encapsulation and it may oxidize in presence of heat, moisture and air.Finally, decomposition of EVA encapsulation leads to delamination, optical decoupling, metallic corrosion etc. with subsequent module efficiency loss [3].Thus, it is essential to improve the thermal stability of the encapsulating material by blending with 1300 (2024) 012015 IOP Publishing doi:10.1088/1757-899X/1300/1/012015 2 minimum quantity of more thermal resistant polymer and nanoparticle.Nanocomposite is the best option to achieve tailor made properties which is the aim of this research.
Blending of elastomeric EVA with very less amount of Low Density Polyethylene (LDPE) plastomer develops thermoplastic elastomer (TPE) with both flexible and rigid moieties in their phase structure which improves the thermal resistance of EVA.EVA is a copolymer of non-polar ethylene and polar vinyl acetate (VA) monomers and the structure of non-polar ethylene backbone is similar with LDPE.These two polymers have different structural polarity and degree of crystallinity and they are only miscible in amorphous zone [4].Compounding of amorphous polymer with semi-crystalline polymer can provide immense possibilities in modifying desirable blend properties.Further, inclusion of nanoparticles can enhance the blend structure [5].Generally, charge passes through the polymerpolymer and polymer-nanoparticle interfaces.Interface generation has strong influence on the temperature dependent electrical insulation properties of blend nanocomposites and which again depends on the sequence of components addition [6].The purpose of this investigation is to study the effect of temperature on DC volume resistivity and calculation of activation energy as a function of sequential ingredients addition.

Sample preparation
Solution blending was carried out with 28% VA content EVA and LDPE with FNS in xylene medium at 90 ± 2℃ with two various sequences of ingredients addition.
Sequence 1 -40 g of EVA was initially dissolved in 100 ml of xylene followed by 0.44 g of FSN addition and the mixture was stirred vigorously.After 5 min of stirring 4 g of LDPE was incorporated to the mixture and blending continued for 12 min.Benzoyl peroxide (BPO) was added in to and the solution was stirred for final 5 min.0.5 mm thin film was casted after solvent evaporation by 10 min compression moulding at 130±2℃ and the sample is designated as ELS-1.
Sequence 2 -40 g of EVA was initially dissolved in 100 ml of xylene followed by 4 g of LDPE addition and the mixture was well blended for 12 min.Thereafter, FSN was added to the solution by 1 wt% of total polymer content and mixed thoroughly for another 5 min.BPO was incorporated and the solution was stirred for final 5 min.After solvent evaporation, 0.5 mm thin film was casted by 10 min compression moulding at 130±2℃ and the sample is designated as ELS-2.
Sequence 3 -4 g of LDPE was initially dissolved in 100 ml of xylene followed by 0.44 g of FSN addition and the mixture was well blended for 4 min.Thereafter, 40 g EVA was added to the solution and mixed vigorously for 12 min.BPO was incorporated and the solution was stirred for final 5 min.After solvent evaporation 0.5 mm thin film was casted by 10 min compression moulding at 130±2℃ and the sample is designated as ELS-3.

Transmission electron microscopy (TEM)
Morphology of EVA-LDPE-FSN nanocomposites was investigated under Transmission electron microscope.70 nm thin sample prepared by using an ultramicrotome equipped with 45° cutting edge diamond knife -50 °C in liquid nitrogen atmosphere.The micrographs were taken with JEOL JEM 2100 high resolution TEM (HRTEM) instrument by using LaB6 filament and the accelerated operating voltage was 200 keV having a resolution of 1.9 Å.

Volume Resistivity (VR)
Volume Resistivity (VR) was estimated by using a Sefelec Megohm Meter, model M 1500P and a Weiss technik climatic chamber.Three terminal resistivity probe ETS Model 803B was used to measure VR. 500V DC was applied for a duration of 60 sec to measure VR at 25±℃, 50±℃ and 75±℃ as per ASTM D257-21.The DC electrical resistance measurement set up was depicted in Figure 1.

Results and Discussion
In order to study the effect of mixing strategy on the morphology of the EVA-LDPE-FNS nanocomposites TEM analysis has been carried out and the image presented in Figures 2 (a), (b) and (c).Figure 2(a) represents the bulk morphology of ELS-1 sample; where FNS was primarily mixed with EVA followed by blending with LDPE.EVA and LDPE are partially miscible and subsisted as two-phase systems.Noticeably it is found in ELS-1 sample that maximum FSN interacted with EVA due to initial high Vander-Wall interaction between EVA and FSN.These thermodynamic effect leads to the filler agglomeration in ELS-1 sample based on the affinity of FSN with EVA.A homogenous FSN dispersion has been found for ELS-2 in Figure 2(b) as FSN was added to EVA-LDPE blend.This uniform distribution of FSN nano particles were observed in thermoplastic and elastomeric phases along with the interphase where FSN migration was fully controlled by kinetic effect [7].Moderated agglomerated morphology has been observed for ELS-3 in Figure 2(c).The cluster structure of FSN broke in presence of solvent and could easily migrate into the dissolve LDPE phase.LDPE-FSN does not hold strong physical bonds.FSN nano particle had choice to migrate towards EVA phase through the interphase.FSN percolation rate is lower in ELS3 than ELS1.As a consequence, deagglomerated FSN distribution was developed in ELS3.Temperature dependent VR of EVA-LDPE-FSN nano composites prepared with different mixing strategy are presented in Figure 3. Sufficient electrical insulation in a wide temperature span is essential for PV encapsulating material to prevent leakage current for maintaining module reliability and performance.Figure 3 reveals that all the prepared nanocomposite encapsulating materials have high VR, but with temperature it decreases significantly.The intensity of VR reduction with temperature is dependent on the ingredients mixing strategy.ELS2 sample provided higher electrical insulation than its other two counterparts.ELS1 presented a medium VR mangitude in the studied temperature range.The reason may be FSN is mainly confined in elastomeric EVA phase in ELS-1 sample and with temperature rise the mobility of VA part of EVA is restricted by the agglomerated nanoparticle as presented in schematic diagram Figure 4(a).The estimated retention values of VR are 61.6% and 20.0% at 50 ℃ and 75 ℃ respectively for ELS1.As per Figure 4(a) FSN agglomeration was prominent in EVA network phase which restrict the movement of EVA segment; but EVA starts flowing on melting at 75 ℃ caused less VR retention.Contrarily, FSN uniformly distributed in EVA-LDPE blend of ELS2 sample as depicted in schematic diagram Figure 4(b) due to good thermodynamic and kinetic effects.FSN are localized in the co-continuous EVA-LDPE matrixes and at the interphase and efficiently offered very good VR due to the stabilize morphology [5].Viscoelastic response in more in homogeneous ELS2 system [6].Simultaneously, VR retention are estimated as 21.6% at 50 ℃ and 3.13% at 75 ℃.Whereas, ELS3 sample could not afford good electrical insulation in all the temperatures under this study.This result is correlated with the TEM analysis in Figure 4(c) where FSN was premixed with the thermodynamically incompatible LDPE then blended with compatible EVA phase.This provoked the movement of FSN towards EVA through the interphase [8].As a consequence, interphase is behaved as charge trap and the overall VR dropped [9].The estimated magnitude of VR retentions are 18.5% at 50 ℃ and 4.51% at 75 ℃ for ELS-3.
Figure 5 confirmed that all the ELS nanocomposites have a strong inverse exponential relationship of volume resistivity (ρv) with temperature and the influence of nano particles addition strategy in the polymeric matrices and the chain movements of both polymers especially in EVA is an ionic conduction process and can be controlled by Arrhenius model.Thus, by using Arrhenius equation (Equation 1) the activation energy of ELS nanocomposites can be calculated.
Where, σ is the electrical conductivity which is equivalent to 1/ ρv, σ0 is the pre-exponential factor, T is the absolute temperature, and k is Boltzmann constant.The activation energy, Ea can be obtained from the slope of the logσ versus 1000/T plot.In Figure 5, the ordinate is ln(1/ ρv), abscissa is 1000/T and the slope after linear approximation is equal to -Ea/k.
The estimated values of Ea for all the ELS samples are presented in Table1.It is confirmed that the energy acquired by the nano particle to free itself from the polymer matrix is less for ELS1 and maximum for ELS2.Agglomerated discrete nanoparticle phase in ELS1 has been found from TEM micrograph of ELS1 sample.In agglomerated structure particle-particle collision is more which decreases the energy barrier of the system [10].Whereas, nanoparticles were distributed uniformly in ELS2 and moderately in ELS3.These aided in the increment of activation energy values.

Conclusion
Effects of FSN addition and sequence of mixing in fabrication of TPE nanocomposites were evaluated in this study.The sample films have been characterized by TEM and temperature dependent electrical bulk resistance followed by activation energies measurements.The magnitude of activation energy of ELS samples provided a strong structure-property relationship.FSN contributed a major role in tailoring the morphology of the nanocomposites besides its reinforcing ability.It was also perceived that ingredients mixing strategy is a strong function on the dispersion of FSN in the binary TPE system.Mixing of two polymers followed by FSN addition provided homogenous TPE/FSN nanocomposite as obtained in ELS2 sample.Therefore, ELS2 accorded with better electrical resistance and comparatively superior activation energy.A well balanced thermodynamic-kinetic effect has been established in ELS2.Finally, it can be concluded that the properties of EVA-LDPE blend nanocomposites are significantly ruled by the kinetics and thermodynamics effects by selective localization of nanoparticles in partially miscible blend.

Figure 1 .
Figure 1.Volume Resistance measurement set up at different temperature

Figure 5 .
Figure 5. Arrhenius plot of Ea estimation

Table 1 .
Activation energy of various ELS samples